A drug which exhibits an excellent bioactivity and safety profile when tested in experimental models may be less active and/or more toxic when administered to human subjects. One possible reason for this disparity is that a molecule may be unable to reach target site(s) of action at therapeutic concentrations and/or accumulate at toxic levels in one or more tissues. Such pharmacokinetic differences between in vitro and in vivo models, and between test species and humans, may significantly limit the therapeutic utility of certain compounds, making drug development a challenge.
Physicochemical properties, therapeutically effective dosage, and route of administration, can each influence the pharmacokinetic profile of a drug molecule. The therapeutically effective dosage is fixed for a particular drug. Nonetheless, a change in the route of administration may allow a reduced drug dosage if the new route offers higher bioavailability. For instance, given suitable physicochemical properties, a drug with poor oral bioavailability requiring a high dosage may be formulated for parenteral administration at a lower dosage due to its improved bioavailability. However, a different route of administration is generally possible only if physicochemical properties of a given drug molecule are suitable for the new dosage form. The physicochemical makeup of many existing drugs limits their use to oral administration, resulting in high dosages and poor pharmacokinetic profiles. Accordingly, efforts have been made to modify the physicochemical properties of existing drugs and/or their formulations.
A drug with poor solubility will often exhibit poor bioavailability—a situation which can either hinder the drug development or require administration of high dosages to attain therapeutically effective blood levels of the drug. Tricor® (fenofibrate), for example, was launched as a 300 mg capsule. Particle size reduction to a fine powder increased the solubility of the drug and allowed a dosage reduction down to 200 mg. Addition of a surfactant to the fine powder led to a formulation with a bioavailability similar to the 300 mg and 200 mg dosages using only a 160 mg dosage tablet. Another bioequivalent formulation containing nano-particles of the drug allowed for an effective 145 mg dosage. Thus, a significant decrease in the dosage of Tricor® (greater than 100%) was achieved by increasing its solubility which led to an increase in bioavailability. However, despite some examples of solubility improvements from particle size reduction, the intrinsic conditions of oral administration (e.g., limited aqueous media in the GI tract) may limit the solubility and bioavailability enhancements for certain drugs.
Another technique used to increase solubility is to make molecular complexes of insoluble/poorly soluble drugs with more soluble molecules such as cyclodextrins. Itraconazole (Sporanox®), voriconazole (Vfend®) and zisprasidone (Geodon®) are examples of successful applications of this technique. However, this application generally requires a large excess of cyclodextrin relative to the amount of drug being solubilized and may not impart the desired increase in solubility to the entire drug sample (for instance, a dosage of 10 mg itraconazole, 200 mg of voriconazole, or 20 mg of zisprasidone requires 400 mg, 3200 mg, or 294 mg of cyclodextrin, respectively).
While the importance of discovering new drugs cannot be overstated, the ability to improve the physicochemical properties of existing drugs has it bounties. Therefore, there is still a clear and unmet need for improved drugs, such as prodrugs of existing drugs.
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